MEPs should threaten to block an EU-US trade deal to make the NSA and GCHQ disclose the full nature of their surveillance, the authors said. Photograph: Bloomberg/Bloomberg via Getty Images

The authors of a new study on mass-scale surveillance have accused the intelligence services of the US and EU countries of violating European law and urged the European parliament to take action.

Sergio Carrera, a Spanish jurist, and Francesco Ragazzi, a professor of international relations at Leiden University in the Netherlands, who co-wrote the paper, made the appeal for European action at a hearing in the EU parliament in Brussels on Thursday.

They said the US National Security Agency (NSA), the UK's GCHQ and equivalent bodies in France, Germany, the Netherlands and Sweden had breached basic articles of the EU treaty, such as article 4.3 on "sincere co-operation", as well as privacy clauses in the EU charter of fundamental values and in the European charter of fundamental rights.

They also noted that EU agencies such as the joint police body, Europol, and the EU foreign service's intelligence-sharing branch, IntCen, were in all likelihood using data "stolen" from European citizens.

"It's no longer credible to say the EU has no legal competence and should do nothing on this. Sorry, we don't think this is acceptable," Carrera said.

"We are witnessing a systematic breach of people's fundamental rights," he added.

Ragazzi said: "The bigger the crisis, the more the system of checks and balances should be reinforced. This is what distinguishes democracies from police states."

The idea that espionage is a national prerogative has been widely used to deflect EU queries into the scandal.

The British ambassador to the EU, Sir John Cunliffe, in a letter to the EU parliament last month said the GCHQ chief, Sir Iain Lobban, had no obligation to answer MEPs' questions because "national security is the sole responsibility of member states".

But Carrera and Ragazzi urged MEPs to use "all the powers at their disposal" to break the wall of silence.

They said the EU parliament should threaten to block an EU-US free trade agreement unless the NSA and GCHQ disclose the full nature of their surveillance programmes.

They said MEPs should push EU countries to draft a "professional code for the transnational management of data".

They also called for new EU laws to stop internet companies giving information to intelligence services, to protect whistleblowers such as the NSA leaker Edward Snowden, and to form a permanent oversight body on intelligence matters.

Snowden's revelations show that GCHQ alone hoovers up 21 petabytes of internet data each day, while the NSA has forced private companies such as Facebook to hand over customers' files.

A senior Facebook executive, Erika Mann, was due to attend the EU hearing on Thursday but cancelled at the last minute citing agenda problems.

The head of IntCen, the former Finnish spy chief Ilkka Salmi, spoke in a closed session on Thursday morning, but said only – according to one MEP – that it was "natural" that intelligence services intercepted their own citizens' emails and phone calls.